Fix flatpak build

While only libmm-glib is used by flatpak builds, the systemd/udev files
are installed into a location which does not match the %files listing,
and that wouldn't make sense if they were being used, unless these
arguments are specified.  There is no macro for the udev parent
directory, and the package buildsystem adds rules.d itself, so it ends
up needing to be hardcoded.

ModemManager.spec

@@ -106,6 +106,8 @@ Vala bindings for ModemManager
 # We'll just install the bash-completion file ourselves.
 %meson \
 	-Ddist_version='"%{version}-%{release}"' \
+	-Dudevdir=/usr/lib/udev \
+	-Dsystemdsystemunitdir=%{_unitdir} \
 	-Dvapi=true \
 	-Dgtk_doc=true \
 	-Dpolkit=permissive \